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Ann. Geophys., 27, 461–472, 2009www.ann-geophys.net/27/461/2009/© Author(s) 2009. This work is distributed underthe Creative Commons Attribution 3.0 License.

AnnalesGeophysicae

Characteristics of mesospheric gravity waves near the magneticequator, Brazil, during the SpreadFEx campaignM. J. Taylor1, P.-D. Pautet1, A. F. Medeiros2, R. Buriti2, J. Fechine2, D. C. Fritts3, S. L. Vadas3, H. Takahashi4, andF. T. Sao Sabbas41Utah State University, Logan, UT, USA2Universidade Federal de Campina Grande, Campina Grande, Paraiba, Brazil3NorthWest Research Associates, CoRA Division, Boulder, CO, USA4Instituto Nacional de Pesquisas Espaciais (INPE), Sao Jose dos Campos, Sao Paulo, Brazil

Received: 6 May 2008 – Revised: 4 August 2008 – Accepted: 4 August 2008 – Published: 2 February 2009

Abstract. As part of the SpreadFEx campaign, coordi-nated optical and radio measurements were made from Brazilto investigate the occurrence and properties of equatorialSpread F, and to characterize the regional mesospheric grav-ity wave field. All-sky image measurements were made fromtwo sites: Brasilia and Cariri located ∼10◦ S of the mag-netic equator and separated by ∼1500 km. In particular, theobservations from Brasilia provided key data in relativelyclose proximity to expected convective sources of the grav-ity waves. High-quality image measurements of the meso-spheric OH emission and the thermospheric OI (630 nm)emission were made during two consecutive new moon pe-riods (22 September to 9 November 2005) providing exten-sive data on the occurrence and properties of F-region de-pletions and regional measurements of the dominant gravitywave characteristics at each site.A total of 120 wave displays were observed, comprising

94 short-period events and 26 medium-scale gravity waves.The characteristics of the small-scale waves agreed well withprevious gravity wave studies from Brazil and other sites.However, significant differences in the wave propagationheadings indicate dissimilar source regions for the Brasiliaand Cariri datasets. The observed medium-scale gravitywave events constitute an important new dataset to studytheir mesospheric properties at equatorial latitudes. Thesedata exhibited similar propagation headings to the short-period events, suggesting they originated from the samesource regions. Medium-scale waves are generally less sus-ceptible to wind filtering effects and modeling studies utiliz-ing these data have successfully identified localized regions

Correspondence to: M. J. Taylor([email protected])

of strong convection, mainly to the west of Brasilia, as theirmost likely sources (Vadas et al., 2009).

Keywords. Atmospheric composition and structure (Air-glow and aurora) – Meteorology and atmospheric dynamics(Middle atmosphere dynamics; Waves and tides)

1 Introduction

Large-scale instabilities in the equatorial ionosphere are awell documented plasma phenomena that are know to causesevere regional blackouts in radio communications and tosignificantly disrupt GPS-based terrestrial navigation sys-tems (e.g. Basu et al., 1999). These instabilities develop inthe early evening hours, during the pre-reversal enhancementof the zonal electric field, when strong upward plasma driftscause the F-region ionosphere to rise significantly (Heelis etal., 1974; Fejer et al., 1999). The resultant instabilities growvia the Rayleigh-Taylor mechanism which creates bubbles(or plumes) of plasma which can attain altitudes as high as1500 km at the equator within a few hours (e.g. Woodmanand La Hoz, 1976; Huang et al., 1993). The bubbles are read-ily observed in radar plots and appear as towering plumesextending from the lower ionosphere to the top side (e.g. Hy-sell and Burcham, 2002; de Paula and Hysell, 2004; Batistaet al., 2004; Buriti et al., 2008). As the plumes raise in alti-tude their electron density signatures map down the magneticflux tubes to lower latitudes where they are detected in the F-region airglow emissions as field-aligned plasma depletions(Rishbeth, 1971; Batista et al., 1986; Abdu, 2001).Of key importance to the understanding and eventual

prediction of Spread F phenomena is the identification ofpotential seed mechanisms that can both initiate and help

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462 M. J. Taylor et al.: Mesospheric gravity waves during SpreadFEx campaign

accelerate the growth rate of the Rayleigh-Taylor instability(RTI). A number of theoretical and modeling studies haveshown that atmospheric gravity waves may provide the es-sential seed forcing necessary to describe the observed bub-ble structures and their growth rates (Woodman and LaHoz,1978; Rottger, 1981; Anderson et al., 1982; Valladares et al.,1983; Hanson et al., 1986; Hysell et al., 1990; Huang andKelley, 1996a, b, c; Sekar and Kelley, 1998; Taylor et al.,1998). Gravity waves are generated primarily within the tro-posphere by severe weather disturbances, such as deep con-vection (e.g. Fritts and Alexander, 2003), as well as strongorographic forcing (e.g. Jiang et al., 2004). At tropical lati-tudes deep convection associated with severe storms is a co-pious source of a broad spectrum of gravity waves observedat mesospheric heights (e.g. Taylor et al., 1997; Nakamura etal., 2003; Medeiros et al., 2004; Suzuki et al., 2004; Pautetet al., 2005). These storms develop during the late afternoonand are well placed in space and time to provide seeding forthe plume development over Brazil. The apparent correlationbetween ESF and enhanced equatorial convection providesadditional support for this concept (McClure et al., 1998).Gravity waves propagate energy and momentum upwards

as they grow exponentially in amplitude and are known toplay a key role in the MLT dynamics (e.g. Lindzen, 1981;Holton, 1983; Garcia and Solomon, 1985; Alexander andHolton, 1997; Fritts and Alexander, 2003). The waves arereadily observed at mesospheric heights using the naturallyoccurring airglow emission. In particular all-sky imagingsystems have been used to characterize the spectrum of thesmaller-scale, shorter period waves (horizontal wavelengthλx<100 km, observed period τ<30min), which are knownto contribute significantly to momentum deposition at MLTheights (Fritts and Vincent, 1987). These data also provideimportant directional information helpful for identifying thesources of the waves (e.g. Taylor and Hapgood, 1988; Naka-mura et al., 2003; Pautet et al., 2005). However, recentmodeling studies have shown that a significant fraction ofthe convectively generated gravity waves of medium-scales(λx >100 km and observed periods of a few tens of minutes)are capable of propagating well into the lower thermospherewhere they may directly act as seeds for the RTI (Vadas etal., 2009).The SpreadFEx campaign was designed to investigate the

potential and properties of convectively generated gravitywaves as seeds for spread-F. The campaign was conductedin Brazil during the Austral spring 2005 and employed abroad range of available ground-based radar, ionosonde, all-sky imaging, and GPS instrumentation, located at a numberof selected sites in Brazil (see Campaign overview by Fritts etal., 2009). Here we summarize the results from the airglowimaging instrumentation used primarily to characterize themesospheric gravity wave field near the magnetic equator,and to identify potential medium-scale wave events for de-tailed ray-tracing studies (as discussed by Vadas et al., 2009).

2 Imaging instrumentation

For the SpreadFEx campaign three high-quality imaging sys-tems were used to quantify the occurrence and propertiesof mesospheric gravity waves and F-region depletions overBrazil using coordinated observations of several mesosphericand thermospheric nightglow emissions (see Fechine et al.,2009; Pautet et al., 2009; Vargas et al., 2009; Wrasse et al.,2009). In particular, all-sky (180◦ field of view) image mea-surements provide two-dimensional information with highspatial and temporal resolutions over a large geographic area(e.g. Taylor et al., 1995, 1997a).All three imagers utilized a sensitive back-thinned

1024×1024 pixel charge couple device (CCD) and werefitted with computer controlled filter wheels enabling se-quential measurements of selected airglow emissions. Eachimager was used to measure the mesospheric near in-frared (NIR) hydroxyl (OH) Meinel broad band emission(710–930 nm) and the OI (630.0 nm) thermospheric red-lineemission. The nocturnal OH emission originates from awell-defined emissive layer centered at ∼87 km with halfwidth 8–10 km (Baker and Stair, 1988), while thermospheric630.0 nm is somewhat broader (∼50 km half-width) and re-sides at ∼250 km altitude, slightly below the F-region peakof electron density (e.g. Tinsley et al., 1973). Typical expo-sure times were 15 s for the bright OH emission bands and90–120 s for the fainter OI (630 nm) line emission, with acadence time of typically 2–2.5min. The data were 2×2binned on the chip resulting in a zenith horizontal resolu-tion of ∼0.5 km (e.g. Taylor et al., 1995). At Brasilia ad-ditional measurements of the OI (557.7 nm) line emission(peak altitude ∼96 km) and a background sky measurement(at 572.5 nm) for assessing sky clarity, were also occasion-ally made after 2 October.

3 Coordinated observations

The Utah State University (USU) multi-wavelength imagingsystem was deployed at a field site near Brasilia (14.8◦ S,47.6◦W). The two other imaging systems were located atpermanent sites at Cariri (7.4◦ S, 36.5◦W) and CachoeiraPaulista (22.7◦ S, 45.0◦W), and were operated by the Uni-versidade Federal de Campina Grande (UFCG) and INPE,respectively. Figure 1 illustrates the all-sky observing geom-etry. The location of these sites was chosen to provide goodlatitudinal and longitudinal sampling of the mesosphericgravity wave field while at the same time measuring the oc-currence, spatial and temporal properties of thermosphericdepletions induced by equatorial Spread-F. The USU cameraat Fazenda Isabel (hereafter referred to as Brasilia) and theCariri imager were both located at ∼10◦ S of the magneticequator. This region was deemed of high interest as the ob-served bubble structure corresponded to equatorial plumesextending southward are a relatively common occurrence

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M. J. Taylor et al.: Mesospheric gravity waves during SpreadFEx campaign 463

Fig. 1. Map of Brazil showing the location of the three camerasites; near Brasilia (14.8◦ S, 47.6◦W), at Cariri (7.4◦ S, 36.5◦W)and Cachoeira Paulista (22.7◦ S, 45.0◦W). The circles indicate thenominal fields of view of the all-sky imagers used for the meso-spheric OH gravity wave measurements (∼350 km radius), assum-ing an emission height of ∼87 km, and the thermospheric 630 nmdepletion measurements (∼1000 km radius) for a peak emission al-titude of ∼250 km.

over Brazil during the springtime. Cachoeira Paulista is lo-cated at 15.8◦ Smag and provided sensitivity to higher alti-tude plumes, which are less common, but map to signifi-cantly lower latitudes. Importantly, these three sites also pro-vided critical regional information on the mesospheric grav-ity wave field during the occurrence of Spread-F essential forour combined investigation of the potential for gravity waveseeding of ESF.In particular, the measurements from Brasilia provided

key data in relatively close proximity to expected convectivesources of the gravity waves (located primarily to the Westover the state of Mato Grosso), that may penetrate to thermo-spheric altitudes. The location of this field site was chosenespecially for this campaign and observations were made fora limited ∼2 month period during the spring 2005 spread-Fseason. In contrast, the Cariri and Cachoeira-Paulista siteswere operated on a near-continuous basis by INPE in collab-oration with the Universidade Federal de Campina Grande(for Cariri). Figure 1 shows the nominal fields of view ofthe all-sky imagers used for the mesospheric OH gravitywave measurements (∼350 km radius), assuming an emis-sion height of ∼87 km, and the thermospheric 630 nm deple-tion measurements (∼1000 km radius) for a peak emissionaltitude of ∼250 km. As the Brasilia and Cariri sites are sep-arated by∼1500 km the mesospheric data were more limitedto regional gravity wave studies. However, the much largerfields of view of the thermospheric measurements gave sig-nificant overlap and provided good continuity for investigat-

Fig. 2. Two examples of all-sky images showing short and medium-scale gravity wave structures in the mesospheric OH emission. Thedata were obtained from Brasilia on two consecutive nights (a) 30September–1 October and (b) 1–2 October.

ing bubble dynamics (Pautet et al., 2009; Takahashi et al.,2009).Coordinated image measurements were made during two

consecutive moon down periods from 22 September to 9October, and from 23 October to 9 November 2005. Un-fortunately, poor weather conditions limited the CachoeiraPaulista measurements during the first observing periodwhile the Brasilia measurements were often constrained bylocal thunderstorm activity during the latter part of the sec-ond observing period. The gravity wave measurements pre-sented here utilize the Brasilia and Cariri data sets (total 17nights from Brasilia and 19 from Cariri) when coincidenthigh-quality image data were obtained on 8 nights, for quan-tifying the mesospheric gravity wave characteristics as wellas the occurrence and properties of thermospheric bubbles.Figure 2 shows two example all-sky images of gravity

waves observed from Brasilia. These data are typical ofthe short-period wave events observed from both Brasiliaand Cariri during the campaign. In (a), a well-developed,short-period, quasi-monochromatic gravity wave is evidentin the OH emission extending across the entire field of view.This event was observed at ∼01:30UT on the night of 30September–1 October 2005, and exhibited a horizontal wave-length λx=23 ±3 km, an observed horizontal phase speedvx=32±5m/s and an observed period τ=12±2min. Thewave pattern lasted for 2.25 h and progressed towards theENE (azimuth 62±5◦ N), as indicated by the arrow (note,the dark region to the NW is due to low elevation cloud).Figure 2b shows a different type of wave structure imaged

on the following night (1–2 October). The small-scale wavesare just as extensive but are much less coherent, and thereis clear evidence for a larger scale gravity wave modulationin the image data (with a central bright crest and two adja-cent troughs evident at this time), that is closely aligned withthe smaller-scale waves. The short-period structures exhib-ited λx=14±3 km, vx=42±5m/s (τ=5.6±2min) propagatingalmost due east (89±5◦). In contrast, the larger-scale wave(λx=159±3 km, vx=50±5m/s), had an observed period of

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Fig. 3. Histogram plots summarizing the characteristics of the short-period wave events observed from Brasilia (left) and Cariri (right). Theuncertainties in the small-scale wave measurements are λx±3 km, vx±5m/s, θ±5◦ and τ±2min. Note the different scales.

53±2min (∼10 times that the smaller-scale events), as it pro-gressed on a similar azimuth of 97±5◦ N. Spectral analysis ofthis complex data set also revealed the presence of a secondshort-period wave progression more towards the SE (heading138◦ N) with λx=28±3 km, vx=33±5m/s and τ=14±2min.

The superposition of this wave motion with the 5.6min waveevent results in the more “mottled” appearance of the short-period wave events observed on this night.In parallel with these mesospheric gravity wave measure-

ments, observations of the OI(630 nm) emission recorded the



occurrence and characteristics of F-region depletion struc-tures. Short-period gravity waves and spread-F depletionswere observed on every clear night at Brasilia (total 17nights). The 630 nm image data (not shown) revealed large-scale, field-aligned depletion structures that drifted eastwardduring the course of the night with high initial values, around120m/s, in good agreement with available GPS measure-ments (J. S. Haase, personal communication, 2008), that re-duced to zero by local midnight. The number of depletionsand their horizontal scales and latitudinal extent varied con-siderably from night to night. These characteristics are typ-ical of previous bubble measurements over Brazil (e.g. Tay-lor et al., 1997b; Pimenta et al., 2003; Arruda et al., 2006),and separate investigations comparing the scale-sizes of themesospheric gravity waves and F-region depletions, and thelongitudinal variability in the bubble characteristics are pre-sented by Takahashi et al. (2009); Pautet et al. (2009).

4 Image analysis and results

Two analysis techniques were used to characterize the quasi-monochromatic wave events imaged in the OH data fromBrasilia and Cariri. One focused on quantifying the wave pa-rameters of the smaller-scale (λx<100 km), short observedperiod (τ<30min) waves that were evident within a singleimage as coherent wave patterns (e.g. Fig. 2a), while thesecond method utilized “Keograms” to identify and measurethe properties of coherent, larger-scale waves, that are de-tectable over an extended period of time (typically severalhours). In many prior airglow studies, the focus has usuallybeen on quantifying the smaller-scale wave field (e.g. Tayloret al., 1997a; Medeiros et al., 2003; Ejiri et al., 2003). Asthe small-scale waves (often termed bands) are copious, theyprovide important 2-dimensional information on the domi-nant wave characteristics, pattern orientation and directionof motion helpful for source identification studies. However,for this investigation we also wish to identify longer-periodwave events that are generally less susceptible to wind filter-ing, capable of propagating to higher altitudes (due to theirhigher phase speeds), and are therefore better suited for raytracing studies to identify potential source regions (e.g. Vadaset al., 2009).

4.1 Small-scale waves

Figure 3 summarizes the small-scale wave measurementsseparately for Brasilia (left column) and Cariri (right col-umn), in the form of histogram plots of horizontal wave-length (λx), observed horizontal phase speed (vx) and in-ferred observed wave period (τ ). The OH images were firstcalibrated using the known star background and then pro-jected onto a regular 500×500 km spatial grid using an as-sumed peak altitude of 87 km. The horizontal parametersof the data (λx , vx , τ ) and propagation angle (θ), measured

Fig. 4. Plots summarizing the number of wave events versus thedirection of propagation (summed into 30◦ bins) for Brasilia (left)and Cariri (right). Note the different scales.

from geographic North, were then determined using well es-tablished two-dimensional Fourier analysis techniques (Tay-lor and Garcia, 1995; Garcia et al., 1997; Coble et al.,1998). At Brasilia a total of 32 wave events were detectedon 17 clear nights during the two observing periods. Theobserving conditions at Cariri were more favorable and 88events were observed during 19 nights. Both sites exhibiteda range of horizontal wavelengths with distributions between10–25 km for Cariri and 15–30 km for Brasilia. Compari-son of the observed phase speeds shows a broader range atCariri than at Brasilia, but both data sets exhibit a strongpreference for values around 30–60m/s with a range of typ-ically 20–80m/s. These resulted in relatively short observedwave periods clustered around ∼8–11min (typical range∼5–20min) at both sites. This said, in each case the Cariridata were found to have somewhat smaller median values(λx=17.4 km, vx=43.3m/s and τ=7.1min) compared withBrasilia (λx=23.2 km, vx=50.5m/s and τ=9.0min). Thesemeasurements are typical for small-scale mesospheric grav-ity waves as measured from a number of low-latitude sites inboth hemispheres (e.g. Taylor et al., 1997a; Nakamura et al.,2003; Medeiros et al., 2003, 2004; Pautet et al., 2005).Figure 4 compares the observed horizontal propagation di-

rections for the short-period wave events from both sites.Due to the limited number of events, the data are divided into30◦ bins indicating the number of events as a function of az-imuth. Both data sets show a strong preference for eastwardwave propagation with only 4 events (3%) exhibiting a smallwestward component of motion. Both sites also show sig-nificant meridional (N-S) components of motion. At Brasiliathe wave headings are divided into two main directions ∼Eand ∼SE (with almost equal preference), while at Cariri therange of wave headings is significantly larger ∼NE to S withthe strongest preference for NE propagation. These resultsagree well with previous seasonal azimuthal studies in north-eastern Brazil using the same instrumentation (e.g. Taylor etal., 1997a; Medeiros et al., 2003), and are discussed later.



Fig. 5. Unwarped image of Fig. 2b mapped on a 500×500 km lineargrid. The white lines delineate the rows and columns used to createthe west-east and north-south Keograms, respectively.

4.2 Medium-scale waves

To investigate the larger-scale perturbations in the OH data,which have scale-sizes characteristic of medium-scale grav-ity waves, a sequence of images was used to create aKeogram. This technique was first employed to study the de-velopment and motion of large-scale auroral structures (e.g.Eather and Mende, 1981) but has been used on a number ofoccasions to summarize wave activity in airglow image data(e.g. Swenson et al., 2003). The initial data processing wasthe same as for the short-period waves, removing the starsand unwarping the image onto a square 500×500 km grid.For example, Fig. 5 shows the processed image for the OHdata of Fig. 2b (the dark region in the top left corner is dueto trees). A Keogram is then made by taking the central col-umn, or row, of each processed image and splicing the datatogether to create two time series showing the north-south,and east-west development, respectively. The white lines inFig. 5 delineate the central row and columns used for creatingthe Keograms in this analysis.Figure 6 compares the west-east Keograms for Cariri (a),

and Brasilia (b), for the night of 1–2 October (including thedata of Fig. 5). In each plot, the top border corresponds toeast and the lower border to west, with zenith indicated bythe horizontal dashed line. On this night, airglow data wereobtained for about 10 h at each site. The Milky Way is not re-moved in our image processing and is visible in each plot asa bright curved band initially near the zenith, moving west-ward with time and exiting the cameras’ field of view around

Fig. 6. Example west-east Keograms for (a) Cariri and (b) Brasiliaobtained on the night of 1–2 October. The medium-scale gravitywaves appear as coherent, tilted bands.

01:00–02:00UT (later at Brasilia due to ∼12◦ longitude dif-ference between the two sites). Inspection of the Cariri datashows several coherent linear structures with a clear forwardtilt. These are the signature of a medium-scale gravity wavethat was most prominent from ∼21:00 to 02:00UT. The for-ward tilt indicates the direction of the zonal wave compo-nent (which in this case was eastward), while the angle of tiltyields its zonal speed. By combining together the N-S andE-W Keogram information the parameters of the medium-scale waves are readily determined. For this event at Caririthe medium-scale wave had a λx of 265 km, a vx of 69m/s,an observed period of 64min and progressed on a headingof ∼83◦. For comparison, the Keogram data from Brasilia(plot b) show strong evidence of persistent short-period grav-ity wave activity progressing eastwards (indicated by the for-ward tilted fine scale structure), modulated by a larger-scalewave progressing in the same direction (as evident in Figs. 2band 5). This activity continued for most of the night.In total, 6 medium-scale gravity wave events were ob-

served from Brasilia and 20 from Cariri during the courseof the campaign. The histogram plots of Fig. 7 summa-rize their main properties. In this case the Brasilia (dark)and Cariri (light shade) waves are plotted together to aidtheir comparison. The horizontal scale ranges from ∼100 to350 km with most events exhibiting wavelengths in the 100–200 km range. The velocity data show a peak around 40–80m/s with no events >100m/s. This yielded an observed



Table 1. Summary of medium-scale waves observed over Brasilia. The estimated uncertainties are ±10% for the wave measurements.

Date Event # λx (km) vx (m/s) τ (min) θ (◦ N) Start Time (UT) Duration (h)

30 Sep 1 145.1 73.3 33.9 84.3 02:40 122 Oct 2 71.4 57.7 20.6 90 23:05 2.75

3 158.6 50.2 52.7 96.8 01:03 3.75

4 64 70.4 15.2 145.9 00:31 11 Oct 5 61.4 28.7 35.7 143.4 23:53 3.2523 Oct 6 148.3 27.4 90.2 136.3 01:35 1

Table 2. Summary of medium-scale waves observed over Cariri. The estimated uncertainties are ±10% for the wave measurements.

Date Event # λx (km) vx (m/s) τ (min) θ (◦ N) Start Time (UT) Duration (h)

23 Sep 1 170.4 76.8 37 36.1 22:00 32 91.5 56.4 27.8 36.7 21:50 1.2

24 Sep 3 119.9 52.8 37.9 38.7 22:35 24 240.2 81.7 49 39.4 22:00 2

25 Sep 5 189.1 45 70 40.9 22:00 328 Sep 6 207.8 57.7 60 51.4 21:30 2.529 Sep 7 78.1 40.7 32 135.3 21:00 5.530 Sep 8 135 73.7 30.7 135 22:45 1.25

9 220.4 69.3 53 138.6 22:30 2.510 143 39 61 68 21:10 0.6

1 Oct 11 119.6 48 41 129 21:45 1.7512 265.1 69 64 83.2 21:00 5

3 Oct 13 208.9 71.1 49 82 21:30 3.528 Oct 14 184.8 79 39 43.8 22:30 1.529 Oct 15 177.9 64.5 46 88 21:00 5

16 343.2 67.5 84.7 86.1 22:00 330 Oct 17 128.7 46.6 46 34.4 21:30 1.531 Oct 18 148.6 53.8 46 48 21:00 21 Nov 19 322.7 59.1 91 61.6 22:00 32 Nov 20 105.6 38.3 46 150 22:00 2

period distribution extending from ∼20 to 100min with abroad peak centered around 20–60min. For completenessthe characteristics of each of these events are summarized inTable 1 (Brasilia) and Table 2 (Cariri).

5 Discussion

A primary observational goal of the SpreadFEx campaignwas to characterize the mesospheric gravity wave field inclose proximity to the magnetic equator where thermosphericbubbles form and grow. During the campaign we observeda total of 120 wave events, of which 22% were associatedwith medium-scale gravity waves. Together, these data pro-vide essential information on the scale sizes, occurrenceand directionality of mesospheric waves during the occur-rence of Spread-F. The characteristics of the small-scalewaves agree very well with previous gravity wave studiesfrom north-eastern Brazil using the same instrumentation

(e.g. Taylor et al., 1997a; Medeiros et al., 2004). However,our joint measurements of the medium-scale gravity wavesconstitute an important new dataset on their mesosphericproperties at equatorial latitudes. Medium and large-scalegravity waves are usually observed as ionospheric electrondensity perturbations termed Traveling Ionospheric Distur-bances (TID’s). Their characteristics have been studied ex-tensively using ionosonde and radar techniques (reviewed byFrancis, 1975). Large-scale TIDs are often associated withhigh-latitude magnetic disturbances and can propagate largedistances from their auroral source region (e.g. Davies andda Rosa, 1969), whereas medium scale TIDs exhibiting peri-ods of several tens of minutes are prevalent at mid- and low-latitudes and are thought to play a major role in the develop-ment of Spread-F (e.g. Kelley and Fukao, 1991).To examine the nature of the medium-scale waves ob-

served at both sites, Fig. 8 shows a log-log plot of their hor-izontal wavelengths as a function of their observed periods.The Cariri data are marked by open squares while the six



Fig. 7. Histograms showing the combined Brasilia and Caririmedium-scale wave characteristics.

Brasilia events are indicated by shaded circles. For compar-ison, the Brasilia small-scale wave events (solid circles) arealso included on the figure. These three data sets overlap welland indicate a clear trend for increasing wavelength with ob-served wave period. In particular, the fitted trend line to thesedata (R2∼0.9), yields a power law of the form λh=2.52τ 1.05,which indicates that the dynamic range of the observed wavephase speeds is limited (factor of ∼4) compared with the

Fig. 8. Log-log plot showing the horizontal wavelength versus ob-served period. The data comprise the Brasilia and Cariri medium-scale waves and the Brasilia small-scale waves. The strait lineshows the best fit to the data and agrees well with previous radarand optical studies of Reid (1986).

large dynamic ranges (factor of ∼20) for the observed hori-zontal wavelengths and periods, and does not change signifi-cantly between the small and medium scale gravity waves re-ported in this study. This result compares very favorably witha previous, in-depth investigation of upper atmospheric grav-ity wave signatures using a compilation of optical and radarmeasurements by Reid (1986), who determined a trend of theform λh=3.62 τ 1.06. Moreover, Taylor et al. (1997a) com-pared OH image measurements from the Guara campaignwith Reid’s compilation and determined a similar trend lineof the form λh=3.1τ 1.06 for ∼50 short-period wave events.This campaign was conducted in 1994 fromAlcantara, Brazil(2.6◦ S, 44.2◦W), ∼1300 km to the northwest of Cariri, un-der similar seasonal conditions (September–October 1994).Thus, the wave events derived from the Keogram analysisare consistent with those expected from medium-scale grav-ity waves, and they significantly extend the usual range ofgravity wave measurements by imagers.To investigate the origin of these waves, Fig. 9 plots sep-

arately their observed phase speed versus propagation di-rection for Brasilia and Cariri. Comparison of these plotswith the data of Fig. 4 immediately shows that the mediumand small-scale waves observed at each site both exhibitedthe same azimuthal distributions. At Cariri the medium andsmall-scale waves all progressed eastwards but with a strongpreference for NE motion. A similar result was obtained byTaylor et al. (1997a), using OH airglow measurements dur-ing the Guara campaign, over 10 years prior to the Spread-FEx campaign, suggesting that the observed eastward veloc-ity distribution is a seasonally recurrent phenomenon.In contrast, the velocity distribution at Brasilia indicates

two distinct preferential directions of motion: one due eastand the other approximately SE. Due to the large separationof these two sites (∼1500 km) it is not expected that freely



Fig. 9. Velocity plots showing the azimuthal distribution of the medium-scale waves. Note the similarity with the small-scale wave data ofFig. 4.

propagating short-period gravity waves observed at each sitewould have a common origin (e.g. Hines, 1967). Indeed, inan investigation of short-period mesospheric waves observedfrom two well-separated sites (∼660 km) in Japan, Taylor etal. (1998) determined that the majority of the wave events(∼75%) exhibited quite dissimilar characteristics, suggest-ing a preponderance for more localized wave motions andsources. More recently, Ejiri et al. (2003) have also shownsignificant differences in gravity wave characteristics ob-served from sites separated by ∼1000 km. Closer inspec-tion of the medium-scale waves (Table 1) and the small-scalewave data reveals that southeastward wave propagation dom-inated during the first observing period (22 September to 9October), while eastward wave motions prevailed during thesecond observing phase (22 October to 9 November). Thissuggests that the mesosphere over Brasilia was strongly in-fluenced by local wave sources.

Vadas et al. (2009) have investigated the sources of themedium-scale waves observed from Brasilia using ray trac-ing techniques. These waves are less susceptible to wind fil-tering effects than the many of the smaller-scale waves, dueto their larger scale sizes and somewhat higher phase speeds(typically 40–80m/s), and are therefore better suited for raytracing studies to identify potential source regions. As thereis considerable uncertainty in the background stratosphericand mesospheric wind field during the campaign, Vadas etal. utilized balloon sounding, meteor radar data, and TIME-GCM model wind data (and no wind conditions), to estimatethe geographic location of the source region for each of thesix medium-scale wave events. They concluded that strongthunderstorm convection to the west of Brasilia was mostlikely the source of these waves. This result is illustratedin Fig. 10 which shows a GOES infrared satellite image ofBrazil at 20:45UT on 1 October. The location of Cariri andBrasilia are marked. On this night, medium-scale waves wereobserved at both sites (as discussed earlier in Fig. 6). The

We apologize for being so late sending you these comments. We hope it is still possible to operate a few changes. The original Figure 10 has been modified in the paper by Vadas et al. Can you change it in this paper? If you need it as a separate file, where should we send/upload it? Thanks a lot for taking these comments into account.

1 Introduction Page 1 line 3: known The reference at the bottom of page 1 is now: Vadas, S.L., Taylor, M.J., Pautet, P.-D., Stamus, P., Fritts, D.C., Liu, H.-L., São Sabbas, F.T., Rampinelli, V.T., Batista, P.P., and Takahashi, H.: Convection: the likely source of the medium-scale gravity waves observed in the OH airglow layer near Brasilia, Brazil, during the SpreadFEx campaign, Ann. Geophys, in review, 2008. Page 2 line 1: and lower Page 2 paragraph 3 line 2: exponentially Page 2 paragraph 3 line 6: remove the coma 2 Imaging instrumentation Page 3 line 12: (at 572.5nm) 3 Coordinated observations Page 3 line 18: Cachoeira Paulista Page 3 paragraph 2 line 8: Cachoeira Paulista 4 Image analysis and results Page 5 line 6: …were observed during 19 nights.

Fig. 10. GOES infrared satellite image showing the location of theconvection over Brazil on the night of 01 October at 20:45UT. Notethe absence of convection in the vicinity of Cariri. The stars in theenlargement show the results of ray tracing of the event #2 fromTable 1 (Vadas et al., 2009).

satellite image shows considerable convective storm activitygenerally to the west of Brasilia at ranges of a few hundredkm. The insert shows a magnified image of this region. Inparticular, the star marks the ray-traced location of the sourceof the medium scale wave observed from Brasilia on thisnight (event 2 in Table 1). The two computed locations arefor the model winds (black star), and for a “no wind” condi-tion (white star). The close proximity of the traced positionsto strong localized convection that occurred ∼1–2 h prior tothe observed wave event are consistent with this storm be-ing the most-likely source of the medium-scale waves. SeeVadas et al. (2009) for further details.In contrast, the satellite data show no significant convec-

tive activity in the vicinity of Cariri on this night, yet theKeogram data of Fig. 6a clearly indicate a well-developed



medium-scale wave event (accompanied by smaller scalewaves). Wrasse et al. (2009) have also utilized ray trac-ing techniques to investigate potential sources for some ofthe Cariri medium-scale waves. They determined that thesewaves originated near regions of intense upward air mo-tion producing vertical instability, as derived from NCEP re-analysis data. Thus, while strong thunderstorms were mostprobably the source of many of the gravity waves observedfrom Brasilia, it appears that convection was not a prominentsource for the wave events observed well to the east at Cariri.

6 Summary

The SpreadFEx campaign was designed to help quantify therole of gravity waves in the seeding and formation of equa-torial F-region bubbles and associated Spread-F phenom-ena. As part of this program all-sky image measurementsof mesospheric gravity waves were made successfully fromtwo sites in Brazil located ∼10◦ S of the magnetic equator.The large separation of these sites, ∼1500 km, enabled re-gional measurements of the gravity wave field while at thesame time providing continuous, spatially overlapping dataon the occurrence and properties of F-region depletion struc-tures. The characteristics of the short-period wave events ateach site were found to be consistent with prior image mea-surements, from Brazil and from other, low and mid-latitudesites. However, significant differences in the wave propaga-tion headings at each site indicate dissimilar source regionsfor the two data sets.Novel wave measurements using Keograms have been

used to determine the properties of 26 medium-scale grav-ity waves detected in the OH emission at ∼87 km altitude.At each site the medium-scale waves exhibited similar prop-agation headings to the short-period events, suggesting com-mon origins. These wave are less susceptible to wind filter-ing effects than many of the smaller-scale, lower phase speedwaves, and modeling studies using these data have been usedsuccessfully to identify localized regions of strong convec-tion, mainly to the west of Brasilia, as their most likelysources. Forward modeling studies (not discussed herein)have also shown that some of the medium-scale, convec-tively generated gravity waves were capable of propagatingwell into the lower thermosphere where they may have acteddirectly as seeds for the Rayleigh-Taylor instability develop-ment (Vadas et al., 2009).

Acknowledgements. We wish to thank E. Bataglin, manager of theFazenda Isabel Ranch, for hosting our field measurements nearBrasilia. The SpreadFEx program was supported by a NASA Liv-ing with a Star Program under contracts NNH04CC67C and NAS5-02036 (Principal Investigator: D. C. Fritts). The USU all-sky imagemeasurements and data analyses were supported by a subcontractwith Colorado Research Associates (a division of North West Re-search Associates). The optical measurements at Cariri AirglowObservatory were supported by INPE and the Universidade Fed-

eral de Campina Grande, to whom we are most grateful. We ac-knowledge the use of GOES satellite imagery and thank Pete Sta-mus (CoRA) for his assistance in their analysis. We thank T. Naka-mura and anonymous reviewer for their comments.

Topical Editor U.-P. Hoppe thanks T. Nakamura and anotheranonymous referee for their help in evaluating this paper.

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Characteristics of mesospheric gra vity wa ves near the ...vasha/TaylorSFX_2009.pdf · tw o sites: Brasilia and Cariri located ! 10 " S of the mag-netic equator and separated by !